Various ceramics such as SiC and Si3N4, which have good properties such as heat-resistance, corrosion-resistance and mechanical strength, have been developed so far for structural members of aircraft, spacecraft, nuclear reactors or the like driven under severe conditions. Such ceramics are also used as members of heat exchangers or mechanical seals driven under heavy-duty conditions. Especially, SiC is a suitable material in various industrial fields from aerospace to nuclear power generation, due to its excellent heat- and wear-resistance as well as chemical stability and reduced-activation property in a nuclear environment.
SiC is brittle itself, despite of good high-temperature property with a sublimation temperature higher than 2600° C. In order to overcome poor toughness, a SiC fiber-reinforced SiC-matrix composite (hereinafter referred to as merely “a SiC composite”) has been proposed, as reported in A. Lacombe and C. Bonnet, 2nd Int. Aerospace Planes Conf. Proc. AIAA-90-5208 (1990) and C. W. Hollenberg et al., J. Nucl. Mat., 219, (1995)70-86.
Several methods, e.g. hot-pressing and liquid-phase sintering, have been developed so far for manufacturing a SiC composite. However, it is very difficult to manufacture a SiC composite good of thermal conductivity, density and strength, so that the same steps are necessarily repeated in order to improve properties of the SiC composite. Repetition of the same steps complicates a manufacturing process and raises a manufacturing cost. Moreover, members with complicated profiles cannot be manufactured with ease due to repetition of the same steps. In this meaning, a SiC composite has not been available for industrial application, yet.
Although a polymer impregnation and pyrolysis process, hot-pressing and liquid-phase sintering are regarded as methods, which will be industrially adopted in near future, there still remains the problem that a sintering temperature can not be elevated to a level enough to improve properties of a SiC composite without damages of SiC fiber. Due to the restriction on the sintering temperature, properties of the SiC matrix are not well improved. For instance, when SiC fiber synthesized from polycarbosilane is used for manufacturing a SiC composite, a sintering temperature is necessarily predetermined not higher than 1600° C. even in short-time heat-treatment, since heat-resistance of SiC fiber itself is 1300° C. or so at highest. The sintering temperature below 1600° C. is too low to promote a liquid-phase sintering reaction. Consequently, a heat-treated SiC composite is not well densified and poor of thermal and mechanical properties.